Non-crystalline, occurring in botryoidal, stalactitic or concretionary masses; hardness, 7; specific gravity, 2.65; color white when pure; luster waxy; translucent to transparent on thin edges. In addition to the crystalline form, silica is freely deposited in an amorphous or cryptocrystalline form which has the same properties as quartz, except the crystal faces. This is called chalcedony, and it occurs in seams, cavities and free surfaces. When the surface of a chalcedony deposit is free it has a waxy luster. It is generally very brittle and breaks in a peculiar splintery manner. Like quartz it also has a great many varieties, according to the impurities present. Its wide distribution, hardness, and the manner in which it can be chipped have made this a most important stone in the history of the development of civilization. The early men first broke it into rough tools, such as knives, axes, spear points, etc., and used these as cutting tools, of one sort or another, because they held their edge better than most stones. We apply, to the people who used only these chipped stones as tools, the term “_Men of the Old Stone Age_,” or the period is termed the _Palæolithic Age_. Later men learned how to grind the edge to a smoother outline, and this much shorter period is termed the _Neolithic Age_. The use of flints for the first tools is world-wide, and the American Indian when discovered was still using chalcedony in its rough-hewn state. “There the ancient Arrow-maker Made his arrow heads of sandstone, Arrow heads of chalcedony, Arrow heads of flint and jasper, Smoothed and sharpened at the edges, Hard and polished, keen and costly.” Chalcedony is the proper term to use when the color is white to translucent, in which case the surfaces are usually botryoidal and waxy. Carnelian is chalcedony which is clear red in color and translucent. This is one of the first stones used for ornamental purposes and for engraving. Carnelians with figures engraved on them were used by the Egyptians, Assyrians and The Children of Israel, at least 2000 B.C.; and the Egyptian scarabs of the fifth or sixth century B.C., were often carved from this variety of chalcedony, as well as from jasper and agates. The brownish varieties are termed _sard_. Chrysoprase is an apple-green variety of chalcedony the color being due to the presence of nickel oxide. This is by no means as common as most of the varieties of chalcedony, and was long prized as a gem. Plasma is chalcedony with a leek- to emerald-green color, and the same stone when it has small red spots of jasper in it is termed _blood-stone_, or _heliotrope_. These red spots are said by tradition to be drops of the blood of Christ. Jasper is a deep red chalcedony, the color being due to hematite, which is so abundant as to make it opaque. A brown variety colored by limonite is also called jasper, and even green jaspers are found. In all cases the opaque character is common.